Oil companies are hitting the brakes on a U.S. shale land grab that produced an abundance of cheap natural gas — and troubles for the industry.

The spending slowdown by international companies including BHP Billiton Ltd. (BHP)and Royal Dutch Shell Plc (RDSA) comes amid a series of write-downs of oil and gas shale assets, caused by plunging prices and disappointing wells. The companies are turning instead to developing current projects, unable to justify buying more property while fields bought during the 2009-2012 flurry remain below their purchase price, according to analysts.

The deal-making slump, which may last for years, threatens to slow oil and gas production growth as companies that built up debt during the rush for shale acreage can’t depend on asset sales to fund drilling programs. The decline has pushed acquisitions of North American energy assets in the first-half of the year to the lowest since 2004.

“Their appetite has slowed,” said Stephen Trauber, Citigroup Inc.’s vice chairman and global head of energy investment banking, who specializes in large oil and gas acquisitions. “It hasn’t stopped, but it has slowed.”

North American oil and gas deals, including shale assets, plunged 52 percent to $26 billion in the first six months from $54 billion in the year-ago period, according to data compiled by Bloomberg. During the drilling frenzy of 2009 through 2012, energy companies spent more than $461 billion buying North American oil and gas properties, the data show.

Lost Ranking

Prior to this year, oil and gas transactions ranked among the top two in total deal values every year since 2005, except 2008 when they were fourth. So far this year, oil and gas isn’t among the top five.

The land grab began more than a decade ago when improved drilling methods and a process called hydraulic fracturing, which cracks rock deep underground to release oil and natural gas, opened up new production in previously untappable shale fields.

The rush accelerated in 2004 as more shale fields in North Dakota, Pennsylvania and Ohio were identified, opening new troves of petroleum and the prospect of energy independence in North America.

That shortfall caused Shell to write down the value of its North American holdings by more than $2 billion last quarter. Shell, based in The Hague, paid $6.7 billion for North American energy assets in seven transactions since 2009, according to data compiled by Bloomberg.

The company told investors this month that it expects its North American oil and gas exploration to remain unprofitable until at least next year. “The major acreage deals are behind us now,” Shell Chief Executive Officer Peter Voser said in a conference call with analysts.

BHP said it would cut the value of its Arkansas shale assets by $2.8 billion. During a May 14 conference presentation, CEO Andrew Mackenzie said capital and exploration spending will “decline significantly” to around $18 billion in 2014, and continue to fall after that.

Firms depending on asset sales to help finance drilling may not have enough money to pay for higher oil and gas production, said Eric Nuttall, who oversees C$70 million ($68 million) at Sprott Asset Management LP in Toronto. That could slow output growth, especially as companies try to avoid taking on more debt.

“A lot of companies have let leverage get out of hand,” he said, speaking about Canadian firms.

Those companies that have to sell assets will likely fetch lower prices, said Fadel Gheit, an analyst at Oppenheimer & Co. Inc. in New York. Producers with the highest debt levels that need cash to fund development, such as Chesapeake Energy Corp. (CHK), of Oklahoma City, are most at risk of having to accept lower offers from buyers, Gheit said in a phone interview.

“People do not sell unless they really need the money to invest in better options,” he said.

International buyers that branched into North America in recent years don’t need to buy anything else — for now, said Toshi Yoshida, a partner with law firm Mayer Brown LLP, which advises on cross-border oil and gas deals. A lot of them achieved their primary goals of obtaining a supply of long-term, dollar-denominated commodities and the technology needed to turn shale into energy, Yoshida said.

“They will stay here for a long period of time,” Yoshida said. “They will make additional acquisitions when the time is right.”